This application is related to a co-pending application entitled "Pulsed Injection System" filed together herewith.
The invention relates to detecting pulses in fluid flow.
Thermistors, which are temperature variable resistors, are often used to detect the flow of fluids. A thermistor carrying a heat generating current is placed in a fluid stream, which removes the heat at a rate dependent upon the rate of flow of the fluid. A high flow rate will remove a large amount of heat and thus raise the resistance of the thermistor while a low flow rate will remove little heat and conversely lower its resistance. This principle is illustrated in U.S. Pat. No: 2,728,225 by Skibitzke.
One current method of converting thermistor resistance to flow rate is to place the thermistor in the well-known bridge configuration with other resistors. The thermistor's instantaneous resistance is determined by balancing the bridge with a calibrated variable resistor and calculating the resistance from the known values of the other resistors in the bridge. The thermistor resistance is then converted to flow rate by the application of an experimentally-determined conversion factor.
The bridge configuration is sufficient when only the instantaneous flow rate need be determined. There are other situations however in which it is important to know that a change in the rate of flow has occured rather than to know the precise value of the flow rate itself. One example of this situation occurs in the co-pending application entitled "Pulsed Injection System," which may be used to inject odorants into odorless fuel gases.
It is critically important to detect pulses, or rapid changes, in the rate of such fluid flow in such apparatuses as that disclosed in the co-pending application. Odorless fuel gases, such as propane, pose a danger to users in the event of leaks. Users may be overcome by leaking gases without prior warning of the danger. To provide a danger signal, odorants, such as mercaptans, are injected into the gas as it flows from a storage area. The injection process must be continually monitored to provide assurance that the odorant is acutually being added to the gas. In a digital pulse injection system, the optimum monitoring device is a pulse flow detector in the odorant fluid stream.
It is therfore an object of the invention to fill the need for a thermistor flow detector that senses pulses in fluid flow.